This invention relates to apparatus for measuring the downhole tension applied to a cable supporting a well-logging apparatus in a borehole.
To accurately determine the depth of a well-logging tool suspended at the end of a cable in a borehole, it is necessary to measure the displacement of the cable on the surface and to correct the surface displacement measured by the calculated elongation of the cable. The elongation calculation requires information of the surface and downhole tensions applied to the cable. As a first approximation, it is possible to take, as the downhole tension, the apparent weight of the apparatus in the column of drilling mud. However, for apparatus equipped with pads or centering tools, or for directionally deviated wells, the downhole tension depends greatly upon the friction of the apparatus against the borehole wall. It is desirable to measure downhole tension in situ by means of a tensiometer placed between the cable and the well-logging apparatus.
A United States Pat. No. 3,497,958 to Gollwitzer describes a method for determining the depth of a well-logging apparatus using a measurement of the movement of the cable and the values for the surface and downhole tension of the cable. The Gollwitzer patent also schematically describes a device for downhole tension measurement using strain gauges.
A major problem encountered in the design of devices for measuring downhole tension is the difficulty of correctly carrying out the measurement under the temperature and pressure conditions prevailing in boreholes.
It is an object of the present invention to provide a downhole tension measurement device particularly well suited to the difficult temperature and pressure conditions encountered in boreholes.